The Washback

Washbacks at
Glenturret

The washback is where all the alcohol that finds its way into a
distillery's whisky is produced. Later stages may concentrate this alcohol as
it passes through the wash still and then the
spirit still, but its existence in the first
place is entirely due to the action of the yeast in the washback.

If the mash tun can be thought of as the
tea pot in the distillery, then the washback is (or, more usually, the washbacks are) the brewery. The sugary liquid,
or wort, from the mash tun has been collected in the
underback. From there it passes through a cooler to ensure it doesn't kill the
yeast and is pumped into the washback at a temperature of about 23°C.

The washback is a large container usually around 12ft across and often 20ft
or more deep. The depth is not obvious on most distillery tours as the
washbacks are usually seen from a working floor placed about 4ft from their
top. But find a vantage point from which you can see beneath the floor and the
true size of the washbacks becomes apparent. Those at
Highland Park were used as
communal baths by servicemen from the naval base at
Scapa Flow during WWII. The effect of
this on the taste of immediate post-war production has gone unrecorded. (Continues below image...)

Stainless Steel Washbacks at Glenfarclas

After the washback has been two-thirds filled with wort at
23°C, yeast is added, and the brewing process begins. This differs from
beer production in one important respect. At no point is the wort boiled
(unlike in the production of beer) so as the yeast is busily converting sugars
into alcohol in the washback a range of other complex reactions are also going
on, carried over from earlier stages in the process: in particular any
remaining starch in the wort is still being converted to sugar during the
process.

Fermentation continues for between 2 and 4 days. Little seems to happen
at first, but then the yeast really kicks into action and the effect can be
tumultuous, rattling and shaking the washback and its surrounds and threatening
to overspill the washback with active froth. This is often countered by the switcher,
a rotating arm designed to skim the froth before it reaches the top of the
washback. Elsewhere the overspill problem is sometimes tackled by closing the lids and placing
large bricks on them.

Taking too close a look inside a washback at this point in the
process can be a literally breathtaking experience. The action takes place
below a blanket of carbon dioxide that has displaced all the oxygen in the
washback, producing a sensation in the top of the nose like every fizzy drink
you've ever consumed, all at once. Most distilleries use carbon dioxide extractors to
prevent too much escaping into the distillery itself. After fermentation is complete the vaguely
beer-like wash, which has an alcohol content of about 7-8% by volume, is pumped from the washback and into the
wash still for its first distillation.

As the images on this page make clear, there are two very different types of washbacks in use, those made of
wood (often larch or Oregon pine) and those made of stainless steel. The choice appears to be partly down to tradition. Once a distillery
has established its process using a particular approach, it is reluctant to change anything in case that
also changes the character of the end product, and some distilleries that have changed from wooden washbacks to stailess steel
have reported changes in the end product as a result. Beyond that, it appears to be a choice between the
much easier to clean qualities of stainless steel on the one hand, or the better heat insulation
properties of wood on the other, which tends to make it easier to maintain a constant temperature in a wooden washback.